1. Field of the Invention
This invention relates generally to the field of medical imaging systems. In particular, the invention relates to an image detector, for example, for X-ray or computed tomography systems.
2. Related Art
X-ray systems produce 2-dimensional planar images, while Computed Tomography (CT) systems produce 2-dimensional sectional images, sometimes referred to as “slice” or “tomo” images. Although there is interest in obtaining CT style images in X-ray systems, particularly those used for vascular imaging, the detectors used for the two types of systems are significantly different. As a result, for reasons noted below, X-ray detectors are not well-suited to CT imaging and CT detectors do not lend themselves to X-ray imaging.
X-ray detectors are generally flat, or nearly flat in the case of detectors implemented as image intensifier tubes. As a result, the scintillator crystals that absorb the X-rays lie at varying distances and angles with reference to the X-ray source. As examples, pixels at the center of the flat detector are closer to the X-ray source and receive the X-ray beam straight-on. The pixels at the periphery of the detector receive a slightly attenuated X-ray beam at an angle. X-ray detectors using Image Intensifier tubes have curved input surfaces, but the orientation of the curvature is opposite to what would be desired for optimal image quality (the input surface of the vacuum tubes must be domed toward the patient to prevent collapse from atmospheric pressure while using the least possible structural material to minimize beam attenuation).
On the other hand, CT detectors are assembled in geometric shapes, typically circular arcs using a bulky and unwieldy structural frame. In that regard, the CT detector assembly was built as an arc shaped array of flat detector elements that used a many-sided polygon to approximate an arc of constant radius. In past, each CT detector element routed signals out of the detector element only through the top and bottom sides of the detector element (top and bottom in this case referring to orientations parallel to the axial direction of the detector). As the number of elements increased, the wire density along the top and bottom edges of the detector greatly increased. As a result, expanding the size and capabilities of a CT detector beyond a handful of detector elements became very difficult.
The differences in detector shape lead to differences in image processing steps applied after an exposure. Some X-ray systems, for example, implement geometric correction in the form of anamorphic optics in their video image capture components. These optics correct for image distortion arising from the adverse curvature of Image Intensifier tubes. In the case of flat X-ray detectors, there is a loss of image quality at the periphery of the detector relative to the center. The image quality loss arises because X-ray photons that should all be detected by a single peripheral pixel will, due to their transit at an angle through the thickness of the scintillator material, pass through the scintillator material above two or more pixels. As a result, the photons' contribution to the image is distributed (blurred) over more pixels than would be the case if the periphery of the detector were oriented perpendicular to the X-ray source, as are CT detector elements. Even image correction algorithms cannot fully compensate for the image quality degradation due to this geometric problem. When flat-panel X-ray detector image data is used with CT image reconstruction algorithms, this loss of image quality is also present in the CT images so obtained. Thus, the detectors for the two types of systems are very different, making it difficult and cumbersome to incorporate, for example, CT imaging in an X-ray system.
Therefore, there is a need for an image detector that overcomes, at least in part, the difficulties set forth above and others previously experienced.